Method and solutions for treating metal surfaces



United States Patent Ofi 3,189,489 Patented June 15, 1965 ice 3,189,489 METHOD AND SOLUTIONS FOR TREATEJG NIETAL SURFACES Louis Schifiman, Elkins Park, Pa., assignor to Amchem Products, Inc, Ambler, Pa., a corporation of Delaware No Drawing. Filed Sept. 24, 1962, Ser. No. 225,8

. 10 Claims. (Cl. 148-62) This invention relates to the art of coating metal surfaces, and more particularly it relates to the art of applying, through use of stable coating solutions, pro-paint polymeric protective and adhesion promoting coatings to ferriferous, zinciferous and aluminum surfaces proir to the subsequent application of siccative type finishes.

Many different types of coatings have been proposed for treating metal surfaces. ical conversion coatings of the type which are obtained with the use of solutions containing such agents as acid phosphates and/ or chromates. Others embody the addition of resinous vehicles to aqueous acid solutions containing such agents as chromic acid or salts thereof.

Regardless of the system employed, the ultimate objective remains unchanged, i.e., the desire to secure maximum corrosion protect-ion of the metal substrate While simultaneously realizing high levels of paint adhesion, im-

pact resistance and flexibility. That the ability to secure such a desirable combination of properties has proven difficult is evident from the host of literature, particularly patent literature, available in this field.

The principal object of the present invention is the prm Vision of improved methods and solutions for applying resinous, protective, flexible and impact-resistant paint bonding coatings to ferrifer-ous, zinciferous and aluminiferous surfaces.

A further object is to provide articles having a final siccative finish which are greatly improved as to appearance, corrosion resistance and longevity. V

In my co-pending application Serial No. 61,398, filed October 10, 1960, now US. Patent No. 3,063,877, which disclosure is hereby incorporated into this specification and made a part hereof, I disclosed a method of coating metal surfaces through use of an aqueous acid solution containing chromium compounds, which solution com prises not only hexavalent chromium but also reduced forms thereof. Such a solution of chromium compounds, which hereinafter will be referred to simply as mixed chromium compounds, is obtained by the reaction of an aqueous solution consisting essentially of from 50 to 800 g./l. of a hexavalent chromium compound, expressed as CrO with from 0.03 to 1 mol of formaldehyde, expressed as HCHO, for each mol of said hexavalent chromium compound. Coatings resulting from such a solution provide high levels of corrosion resistance on either coated or bare metal surfaces.

In the procedure just described it is preferred that sufficient formaldehyde be reacted with the chrom-ic acid (CrO to reduce at least 5%, and preferably at least 20%, of the total hexavalent chromium present. After the reaction it has been found that the upper limit on the amount of total chromium present in the reduced form is about 60% and that this holds true even though large excesses of formaldehyde and high temperatures during' or following the use of formaldehyde are employed. As pointed out in my previous application referred to above,

Some of these include chem example, when a 33 percent CrO solution is reacted with formaldehyde, using the formaldehyde/chromic acid mol ratio of 0.03 to 1, there results a solution with about 5% of the chromium in the reduced form. As an illustration of reaction ratios yielding a higher percentage of reduced chromium, I have found that by starting with the same 33 percent CrO solution and reacting this solution with formaldehyde utilizing a formaldehyde/chrome acid mol ratio of 0.1 to l, a solution containing about percent of the chromium in reduced form is obtained. Thus, it cen be seen that the use of a low formaldehyde/chromic acid ratio yields lower total reduced chromium. In addition, it has been found that decreasing concentration of the chromic acid results in lower reduced chromic values.

reduced state may be increased by utilizing more concentrated chromic acid or a higher formaldehyde/chromic acid ratio.

Due to the limitation on the maximum amount of chromium which can be reduced by the technique just described, it is not necessary, in general, to use more than about 1 mol of formaldehyde for each mol of hexavalent chromium. If higher mol ratios are used the resultant product may tend to gel on standing. Such gel formation causes no apparent harmand may be broken by vigorous stirring.

It should be noted that, since the reaction of formalde-Z hyde with vchromic acid is exothermic, it is generally preferred to add aqueous formaldehyde solution to the chromic acid because this procedure permits close control of the temperature and it is best to operate with-out raising the temperature above 180 F.

I have now made the surprising additional discovery that if there is added to an aqueous solution containing at least 0.25% by weight of mixed chromium compounds (as hereinabove described), at least 0.25% by Weight of a polyvinyl alcohol compound, and the resulting admix ture is subsequently applied to metal substrates and dried, there results on the treated metal substrates coatings which have improved flexibility, paint adhesion and impact and corrosion resistance.

The polyvinyl alcohol compounds found to be suitable for use in accordance with the teachings of this invention may be any of the commercially available types or grades,

' typical examples of which are those sold under the trade names of Elvanol, Vinol, Lemoflex, and Gelvat-ol. These products include polyvinyl alcohols having viscosities ranging from 4 to cps. (using 4% aqueous solutions at 20 C.), and having degrees of hydrolysis ranging from as low as 60-65% to essentially complete hydrolysis.

As noted hereinabove, the minimum amount of mixed chromium compounds which must be present in the solutions of this invention must be 0.25% by weight of the coating solution. Where less than 0.25% by Weight of these mixed chromium compounds is utilized, the subsequently produced coatings will not provide the increased corrosion resistance otherwise obtainable by the use of the teachings of this invention.

So far as the polyvinyl alcohol content is concerned,

this must be present, as noted hereinabove, in an amount of a least 0.25% by weight of the coating solution. If: less than 0.25% of the polyvinyl alcohol is 'utili'zed, no

substantial improvement in coating properties will be realized; So .far as the upper limit of polyvinyl alcohol is concerned, it has been found that no additional improvements over those obtained initially are experienced by the use of more than about 5% by weight of polyvinyl alcohol based upon the weight of the coating solution.

A preferred upper limit of 2% (by weight based upon the weight of the coating solution) of polyvinyl alcohol has been found on the basis of optimum results consistent with maximum economy of operation. The actual of the polyvinyl alcohol compound employed.

While the amount of mixed chromium compounds used 7 7 may he as much as parts thereof for each part. of poly.-

aqueous solution of mixed chromium compounds there is 7 no apparent need to observe cooling precautions. However, the type of water used to dilute the mixed chromium.

compound solutions, or to dilute admixtures of polyvinyl alcohol and mixed chromium compound solutions should notbe what is customarily termed hard water, since a measure of precipitation sometimes occurs when using same While many natural waters are entirely suitable for use in effecting dilution of either the mixed chromium' compound solutions,'or mixtures thereof with the polyvinyl alcohols, it is preferred, in the interest ofavoiding any possible precipitation of the coating constituents to utilize eitherdeionized ordistilled water supplies. Where distilled or de-ionized'water. sources are not available,

is within the purview of'this. invention to incorporate into these coating solutions from 001 to 0.1% by weight Use of such agents facilitates Wetting of the'rnetal surfaces to .be'treated. While various cationic, anionic or" non-ionic wetting agents have 7 amount used is influenced in part by type and viscosity 7 thereof a wetting agent.

taining heated coating baths and heating equipment. However, if desired, the improved coatings of this invention may be applied at elevated temperatures with no deleterious effects upon resultant quality. V The solutions may be applied to metal surfaces by dip, spray or roller coating techniques, the important consideration being thatthe entire surface area bethoroughy wetted by thecoating solution. In thislatter respect, it

been found to be suitable for use in this. respect, the

r preferred types are the nonicnics, typical of which are the polyethoxylated alkyl phenols containing from .6 to

'12 ethylene groups, and from 8 to 9 carbon atoms in the alkyl group. a V

a Following applicationof the solutions "of this invention to base metal surfaces it isnecessary to subject the treated surfaces to a drying stage, The drying stage serves to cure the mixed organic-inorganic coating and the duration of such drying period will of course depend upon the temperature utilized. .If the temperature is rather low a relatively long period of time will be necessary; to

effect a complete cure, whereas, if the temperature is somewhat elevated, the drying and curing can be accomplished in less time. It has been found that a preferred'rangeof temperatures fortthe drying operation lies anywhere'from about 200 to 350 F.

A coating produced in the manner just described is ex? V tremely usefulper se and for some purposes will be all that is required in order to provide the proper level 'of it is' then recommended that thesolution pH be main- 1 V tained'below about 2.3 since. this pH level has been found to minimize substantially or to eliminate precipitation tendencies. .The pH control may be accomplished through'the addition of small'increments of acids such as phosphoric or formic, and optimum blending is obtained when brisk' agitation is employed throughout the; mixing operation. a a

If desired, compatible co-solvents may be added to the coating solutions of the present invention in order to been found tobe suitable for use'in this respect" include aliphatic alcohols and ketones containing up to four carbon atoms, 7 and aliphatic ether alcohols containing from 4 to 8 total.

carbon atoms. Typical examples of these are the methyl,

ethyl and butyl alcohols, acetone, methylethyl ketone, and

the mono and diethyl, propyl and butyl ethers of ethylene The amount' of co-solvent to' be employed; may range from asli-ttle as 0.1% to 20% based upon theweight of to cure completely the coating oft-the present'invention,

V V ments are intended to be illustrative only, and are not to; improve the spreadability and viscosity-flow characterbe construed as in any way limiting the scope of this inistics thereof. Such co-solvents which have corrosion protection, However, a sicc-ative'finish is'generally applied to the coated surface and where this is (I desired the following technique may be employed. The.

coated surface, preferably, is at least surface dried, but

before drying is sufiiciently complete to effect the required cure, the siccative film may be applied, followingwhich a single baking step may be employedwhich will serve while simultaneously effecting the customary baking of the siccative finish. r

. 'The'inve'ntion will now be described in detail with ref- 7 erenceto specific embodiments thereto, which embodivention.

application of the coatings, each panel was baked for five minutes at 350 F. After cooling, each panel was painted with a stan dard automotive enameland baked to cure the coating solution. .Generally, however, the use of f from 0.1 to about 5% by weight of the coating solution will provide a maximum improvement of flow character istics-to the coating solutions.

' Application of the stable'polyvinyl alcohol-mixed chro- I miurn compound solution should be made on reasonably clean metal surfaces.- present on such metal to be treated, it is preferableto.

Jemploy coated is heavily coated with oil or dirt, a stronger clean- Where 'only slight'oil films" are a solvent .clem ing agent; If the metal to be ing agent, such as'for example, a sodium 'orthosilicate solution; isjpreferably' employed. However, since the cleaning stage forms no part of the present invention, it

will sufiice to state that conventional cleaning operations 'will be adequate in freeing the metal surfaces of contaminants. a r

The coating application is preferably carriedout at room temperature, thereby obviating the need for main same.

' Corrosion tests reported include: 7

(a) Salt spray 336 hours) ASTM-'l3117-.-57T) (b) Humidity (JAN 5-792 (c) Adhesion-Conical Mandril (ASTM-DI522,41)

(d) ImpactThe test panel is impacted by a falling V ball with a given force measured in inch-pounds, de-

forming the test specimen. After impact, the. deformed surface is inspected for loose or cracked paint,- usually on the reverse side of the impact,- and rated 'desc'riptivelya The degree of deformation, and accordingly the severity a. V of any given force of impact is dependent upon the thick-' 7 ness of the specimen' Normally specimens heavier than I 16 gage (0.0625) are not usedgfor the maximum impact available, 86.7 inch-pounds, causes little deformation to these heavy sections; 7 I

The abbreviation MCCl used in the following" table comprises mixed chromium compounds (in accordance with U.S. application Serial No. 61,398 filed October .10, 1960). The total chromium content within the mixed chromium compounds was calculated as CrO for pure 5. The method of claim 1 wherein the pH of the treating solution is maintained below about 2.3 by adding, as

Table I Polyvinyl alcohol Corrosion test results Example N0. Metal coated Pfielt (a) (b) (c) (d) Type Percent inch scribe failure JAN H- ASTM D- Adliesion, impact in ASTM B-1l7- 792 522-41 (65 inch-lbs.)

None Elvanol 7260 0. 25 Failed. None Elvanol 72-50. 2 Do. *1 Elvanol 7260 2 6 0. "2 Elvanol 72-60 r 1 6 Passed. 0. 25 Elvanol 72-60- 0. 25 9.5 Do. 0. 4 Elvanol 72-60 0. 9. 5 D0. 1 Elvanol 51-0 0.25 9. 5 Do. 1 Elvanol 70-7 0. 5 Do. 2 Vmol KR-3l0- 0. 5 9. 5 D0. 2 Viuol KR-3l() 1 9 D0. 2 Lemoflex 2913 2 9. 5 Do. 4 Vinol KR-3l0. 0. 5 9. 5 D0. 4 Elvanol 71-30 1 9 Do. *1 Elvanol ol-O 2 Failed. 1 Elvanol 5l05 2 10 Yassed. 2 Elvanol 51-05- l 10 Do. 1 Elvanol 7260 2 10 D0. 2 Elvanol 72-60 1 9. 5 D0.

*Used CrOa rather than the mixed chromium compounds.

NorE.-E1vano172-60 is a high viscosity product reputed to be essentially completely hydrolyzed. Elvanol 51-05 is a low viscosity product reputed to be hydrolyzed to approximately 88%. Elvanol 70-05 is a low viscosity Lemofiex 29-13 and Elvanol 71-30 are medium viscosity products and are r From the above results it is apparent that the use of mixed chromium compounds, same containing both hex- 30 avalent and reduced forms of chromium in combination with polyvinyl alcohol, provides enchanced flexibility and paint adhesion, as well as impact and corrosion resistant properties to base metal surfaces, when such surfaces, are

treated in accordance with the teachings of this invention. 3

I claim:

1. The method of applying a coating to the surface of metals selected from the class consisting essentially of iron, zinc and aluminum which method comprises treating the surface with an aqueous bath, the coating producing 49 ingredients of which bath consist essentially of:

(1) mixtures of chromic acid including its soluble salts and reduced forms thereof such as are derived from the reaction of an aqueous solution consisting essentially of from 50 to 800 g./l. of a hexavalent chromium compound, expressed as CrO with from 0.03 to 1 mol of formaldehyde, expressed as HCHO, for each mol of said hexavalent chromium compound, and

(2) a polyvinyl alcohol compound the quantity of each of (1) and (2) in the bath being not less than 0.25 by weight, the treatment being continued until the surface is completely Wetted by the solution, and then drying the surface.

2. The method of claim 1 wherein the mixed chromium product reputed to be essentially completely hydrolyzed. Vinol Kit-310, eputed to be essentially completely hydrolyzed.

required, small amounts of acid from the class consisting essentially of phosphoric and formic.

6. The method of claim 1 wherein the treating solution is employed at average room temperature.

'7. The method of claim 1 wherein a siccative film is applied to the surface prior to the drying step.

8. The method of claim 6 wherein the drying temperature is approximately 200 to 350 F.

9. The method of claim 7 wherein the drying temperature is approximately 200 to 350 F.

10. A solution for treating metal surfaces selected from the class consisting essentially of iron, zinc and aluminum, said solution consisting essentially of at least 0.25 by Weight of a polyvinyl alcohol compound and at least 0.25% by weight of reaction products derived from the admixture of an aqueous solution consisting essentially of from about 50 to about 800 grams/liter of hexavalent chromium, expressed as CrO with from about 0.03 mol to about 1 mol of formaldehyde, expressed as HCHO, for every mol of hexavalent chromium, the balance of the solution being Water.

References Cited by the Examiner UNITED STATES PATENTS 2,162,618 6/39 Izard 117-6 2,291,854 8/42 Whyzmuzis 1l4-6.l6 X 2,464,596 3/49 Mason l486.2 2,438,651 11/49 Whiting 148-616 2,768,104 10/56 Schuster et al. 148-62 3,053,692 9/62 Pocock 148-6.2 X 3,053,785 9/62 Rosenbloom 117l32 X 3,063,877 11/62 Schiffman l48--6.16 X

RICHARD D. NEVIUS, Primary Examiner. WILLIAM D. MARTIN, Examiner. 

1. THE METHOD OF APPLYING A COATING TO THE SURFACE OF METALS SELECTED FROM THE CLASS CONSISTING ESSENTIALLY OF IRON, ZINC AND ALUMINUM WHICH METHOD COMPRISES TREATING THE SURFACE WITH AN AQUEOUS BATH, THE COATING PRODUCING INGREDIENTS OF WHICH BATH CONSIST ESSENTAILLY OF: (1) MIXTURES OF CHROMIC ACID INCLUDING ITS SOLUBLE SALTS AND REDUCED FORMS THEREOF SUCH AS ARE DERIVED FROM THE REACTION OF AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF FROM 50 TO 800G./L. OF A HEXAVALENT CHROMIUM COMPOUND, EXPRESSED AS CRO3, EITH FROM 0.03 TO 1 MOL OF FORMALDEHYDE, EXPRESSED AS HCHO, FOR EACH MOL OF SAID HEXAVALENT CHROMIUM COMPOUND, AND (2) A POLYVINYL ALCOHOL COMPOUND THE QUANTITY OF EACH OF (1) AND (2) IN THE BATH BEING NOT LESS THAN 0.25% BY WEIGHT, THE TREATMENT BEING CONTINUED UNTIL THE SURFACE IS COMPLETELY WETTED BY THE SOLUTION, AND THEN DRYING THE SURFACE. 